Engine valve control for braking operation



United States Patent [72] Inventor Frederick Siegler Los Angeles, California [21] Appl. No. 751,540 [22] Filed Aug. 9, 1968 [45] Patented Dec. 15, 1970 [73] Assignee White Motor Corporation Cleveland, Ohio a corporation of Ohio [54] ENGINE VALVE CONTROL FOR BRAKING OPERATION 24 Claims, 13 Drawing Figs.

. [52] U.S. Cl 123/97, 123/90 [51] Int. Cl ..F02d 13/04, F02d 13/06 [50] Field ofSearch l23/97(B), 97,I05,l07,l09,Ill,114,198(F), 90(K1,l(2,l(3)

[56] References Cited UNITED STATES PATENTS 2,002,196 5/1935 Ucko 123/97 3,220,392 11/1965 Cummins 123/97 3,332,405 7/1967 Haviland 123/97 3,367,312 2/1968 Jonsson... 123/97 3,405,699 10/1968 Laas 123/97 3,439,662 4/1969 Jones et al. 123/97 Primary Examiner-Wendell E. Burns Attorney-Smyth, Roston and Pavitt ABSTRACT: An internal combustion engine is temporarily operated as a brake when required by cutting off the fuel supply and holding either the exhaust valves or the intake valves at least partially open throughout a braking period. For this purpose, suitable means is employed to block the return movement of the cam-actuated rocker arms that normally periodically force the corresponding valves open in opposition to the valve springs. Each of the blocking means is operated by a corresponding piston in a corresponding hydraulic chamber. When pressurized hydraulic fluid is supplied to all of the hydraulic chambers to initiate a braking period, the various blocking means advance in sequence as permitted by the sequential operation of the rocker arms until the return movements of all of the rocker'arms are blocked, the chambers then functioning as hydraulic locks for the duration of the braking period.

ENGINE VALVE CONTROL FOR BRAKING OPERATION BACKGROUND OF THE INVENTION the crank shaft to cause the exhaust valves-to open at or near-- the ends of the compression strokes of the pistons. A disclosure of a control system for this purpose is found in the Cummins US. Pat. No. 3,220,392. 1

A more recent development is the concept of obtaining such a braking operation of an internal combustion engine by simply holding either all of the exhaust valves or all the intake valves at least partially open continuously throughout a braking period. This concept is an advance in the'direction of sim: plification in that it eliminates the need for cyclically opening the valves during the braking operation. All that is needed is to block the closing movements of the various valves after they are opened by the corresponding rocker arms.

A problem arises in that any given-moment some of the valves may be completely closed, some of the valves may be completely opened and the rest of the valves may be in the act.

of opening or the act of closing. Consequently, if a plurality of means is provided to block the closing movements of the valves of the respective cylinders, the blocking means must be moved to their blocking positions in sequence and the sequence that is required will depend on the particular point in the normal engine cycle at which the braking action is initiated. a

Another problem is to provide such a braking system that may be readily applied to any type of internal combustion engine without drastic modification of the engine itself. In the various control systems disclosed in the Cummins patent, extensive modification and reconstruction of an engine is necessary to make it possible for the engine to function as a brake. Any attempt to convert an engine to braking operation after the engine leaves the factory would involve extensive rebuilding of the engine.

A further problem is to provide a control system that is relatively simple and relative inexpensive. The mechanisms disclosed in the Cummins patent are complicated by numerous working parts and are correspondingly expensive.

The present invention is directed tothese various problems that are involved in converting an internal combustion engine to braking operation wherein the braking operation is achieved by holding the exhaust valves continuously at least partially open.

' SUMMARY OF THE INVENTION When an exhaust valve or intake valve of a cylinder reciprocates between its open and closed positions, means exterior of the cylinder that is operatively connected to the valve also reciprocates and the invention accomplishes its purpose by partially blocking the reciprocation of such an exterior means. The exterior means may, for example, be the external end of the valve stem or may be a rocker arm that acts on the end of the valve stem.

The preferred embodiment of the invention provides blocking means that may be: advanced from a normal retracted position along the path of oscillation of a rocker arm to block-the return movement of the rocker arm that is actuated by the valve spring. In this regard a feature of the invention is the concept of rotatably mounting a blocking means adjacent the corresponding valve rocker arm on the same axis of rotation as the rocker arm to permit the blocking means to rotate between a normal retracted position and an effective advanced position.

Each of the various rotatable blocking means of a multiple cylinder engine is controlled by a corresponding hydraulic chamber that is hydraulically expanded for the blocking operation. Preferably each hydraulic chamber has a movable wall in the form of a piston and the piston is operatively connected to the corresponding blocking means. Suitable control means places all of the various hydraulic chambers in communication with a source of pressurized hydraulic fluid to initiate a braking operation and terminates the braking operation by venting all of the hydraulic chambers to a low pressure region. ln'the preferred embodimentof the invention, each of the hydraulic chambers is provided with a corresponding check valve to prevent reverse flow from the hydraulic chamber and each of the hydraulic chambers is further provided with a pressure-responsive valve which moves from a normal first position through a second position to a third position in response to rise in pressure in the fluid supply line and vice versa. At the normal first position of the valve the inlet port of the chamber is closed and the release portof the chamber is open to maintain low pressure in the chamber; at the second position of the valve both the inlet port and release port are closed; at the third position of the valve the inlet port is open and the release port is closed to build up fluid pressure in the chamber.

Since each blocking means is independently hydraulically actuated and since each blocking means advances into the path of reciprocation of the correspondingrocker arm, the timing of the advance of each blocking means is determined by the timing of the corresponding rocker arm. Accordingly, there is no timing problem since each blocking means advances only when permitted by the reciprocation of the corresponding rocker arm because the force exerted by the corresponding chamber is not sufficient to overcome the corresponding valve spring. When all of the blocking means are advanced to the desired degree, the check valves cause the corresponding hydraulic chambers to function as hydraulic locks to prevent retraction of the blocking means and thus keep all of the valves at least partially open through the desired braking period.

When the engine is operating at high'speed, oscillation of a rocker arm may occur too briefly to permit the desired quantity of the pressurized hydraulic fluid to flow into the corresponding hydraulic chamber. The provision of a check valve, however, prevents reverse flow from the hydraulic chamber and permits increments of hydraulic fluid to be introduced on the successive oscillations of the rocker arm until sufficient hydraulic fluid accumulates in the hydraulic cylinder to hydraulically lock the corresponding blocking means at the predetermined advanced position.

In the preferred embodiment of the invention, the hydraulic cylinders to control the blocking means of a row of cylinders are relatively small hydraulic cylinders incorporated in the structure of the cylinder head and adjustable screws coact with the respective blocking means to limit their advance and thus limit the degree to which the various exhaust valves are held open.

The features and advantages of the invention may be understood from the followingdetailed description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS tially open positions;

a rotatable blocking means interposed between the two rocker arms;

.FIG. 5 is an end elevation of the structure shown in FIG. 4;

FIG. 6 is a side elevation of the rocker arm for the exhaust valves of the cylinder;

FIG. 7 is a fragmentary plan view of the rocker arm shown in FIG. 6;

FIG. 8 is a front elevational view of the rotatable blocking means;

FIG. 9 is a side elevation of the rotatable blocking means;

FIG. 10 is a diagram of a hydraulic'system for controlling the braking operation; and

FIGS. l1l3 are diagrams representing a hydraulic chamber and three positions of the pressure-responsive valve of the chamber.

I DESCRIPTION OF THE PREFERRED EMBODIMENT In the presently preferred embodiment of the invention shown in the drawings, each cylinder of a multiple cylinder internal combustion engine has a pair of exhaust valves 10 as shown in FIG. 1 and has a similar'pair of intake valves (not shown). Each of the exhaust valves 10 has the usual valve stem 12and is urged to its closed position by a corresponding valve spring 14 that backs against a collar l5 on the valve stem. The upper ends of the two valve stems l2 abut the two arms respectively of a yoke or valve bridge 16 that is slidably mounted on the upper end of a fixed guide post 18. One end of a rocker arm 20 rocks against the valve bridge 16 and the other end is operatively connected in a well known manner to a push rod 22 that is actuated by a cam shaft (not shown).

In the normal operation of the engine for power output the exhaust valves 10 of the various cylinders are opened in sequence by the corresponding rocker arms 20. To convert the engine to braking operation, the fuel supply is cut off and the various exhaust valves 10 are maintained at least partially open for the duration of the braking period.

It is tobe noted that for each cylinder of the engine there are three different external means that are operatively connected to the two exhaust valves 10 of the cylinder to reciprocate or oscillate with the reciprocations of the two exhaustvalves. Thus, the external ends of the valve stems 12 reciprocate with the opening and closing of the exhaust valves; the valve bridge 16 reciprocates with the opening and closing of the exhaust valves; and the corresponding rocker arm 20 oscillates with the opening and closing of the exhaust valves. The basic concept of the present invention is to block the return movement of a selected one of these external means for the purpose of keeping the two exhaust valves from completely closing. In this particular embodiment of the invention, the return oscillation of the rocker arm 20 is curtailed for this purpose.

As indicated in FIG. 4, the rocker arm 24 for the two intake valves of the cylinder and the rocker arm 20 for the two exhaust valves of the cylinder are both rotatably mounted on a short shaft 25 with the opposite ends of the shaft anchored in corresponding fixed upright supports 26. In accord with the invention, a blocking means 28 which may be termed a blocking arm is rotatably mounted on the same shaft 25 between the two rocker arms 20 and 24.

,For the required cooperation between the blocking arm 28 and the rocker am 20, these two arms must be formed with portions for mutual abutment at the advanced position of the blocking arm and for this purpose the rocker arm 20 may be of the configuration shown in FIGS. 6 and 7 and the blocking arm 28 may be of the configuration shown in FIGS. 8 and 9. It may be seen in FIGS. 6 and 7 that the rocker arm 20 has a lateral projection 30 with a rearward face 32 and in FIGS. 8 and 9 it may be seen that the blocking arm 28 has a lateral projection 34 with a forward face 35 to cooperate with the rearward face 32 of the rocker arm. In addition, the blocking arm 28 is fonned with a second forward face 36 to cooperate with suitable stop means. As shown in FIG. 1 the stop means may be an adjustable screw 37 mounted in a overhead bracket 38 which is mounted by cap screws 40 on the two upright supports 26. The adjustable screw 37, which is provided with a suitable lock nut 42 determines the degree to which the exhaust valves 10 are held open by advance of the blocking arm 28.

At the normal retracted position of the blocking arm 28 that is shown in FIG. 2, the forward face 36 of the blocking arm 28 is spaced from the stop screw 37 and the forward face 35 of the blocking arm is retracted from the range of the oscillation of the rearward face 32 of the rocker arm 20. For braking operation of the engine, the blocking arm 28 is advanced counterclockwise to the position shown in FIG. 1 to place its forward face 35 in the path of return movement of the rearward face 32 of the rocker arm 20.

The outer end of the blocking arm 28 is formed with a circular seat 44 for the inner end of a coil spring 45, the coil spring backing against a hollow plug 46 that screws into the cylinder head 48. The outer end of the blocking arm 28 is further formed with a rounded nose 50 which impinges on the outer face of a piston 54. The piston 54 is slidingly mounted in a cylinder 55 in the cylinder head and cooperates with the cylinder to form a variable hydraulic chamber 56.

The cylinder 55 has a relatively wide inner circumferential groove 57 that communicates with a port 58 which in turn communicates with an oil line 60. Throughout the range of reciprocation of the piston 54, the inner circumferential groove 57 is in communication with one end of a diametrical bore 62 in the piston, the opposite end of the bore being closed by a plug 64. Slidingly mounted in the diametrical bore 62 is a pressure-responsive valve member 65 that is movable from a normal first position shown in FIG. 2 through an intermediate position to a third position shown in FIG. 1, the valve member being biased to its normal position by a coil spring 66.

At the normal first position of the pressure-responsive valve member 65 hydraulic fluid may escape from the hydraulic chamber 56 through a bore 68 from the inner end of the piston 54 to the diametrical bore 62 and from the diametrical bore through a second bore 70 to the outer end of the piston. It is apparent that these two bores 68 and 70 constitute a release port for the hydraulic chamber 56.

At the third position of the pressure-responsive valve member 65 that is shown in FIG. 1, the valve member uncovers a passage 72 to the inner end of the piston which passage may be termed an inlet port for the hydraulic chamber 56. In the present embodiment of the invention, this inlet port is provided with a check valve in-the form of a ball 74 that is biased to closed position by a coil spring 75.

In the construction shown, the passage 72 is enlarged and internally threaded to receive a bushing 76 having an axial bore 78, the axial bore being counterbored to form a cage for the coil spring 75. Preferably the bushing 76 is cut away on three sides as indicated in FIG. 4 to provide clearance for oil flow past the bushing.

The manner in which the pressure-responsive valve 65 functions to control flow into and out of the hydraulic chamber 56 thereby to control the movements of the piston 54 may be understood by referring to the three diagrams FIGS. 1ll3. The various parts in the three diagrams represent parts of the actual structure as indicated by the use of corresponding numerals to designate corresponding parts.

The oil pressure in the circumferential groove 57 of the, cylinder 55 is, of course, determined by the oil pressure in the oil line 60. When the oil line 60 is vented to a low pressure zone, the coil spring 66 maintains the pressure-responsive valve member 65 at its normal position shown in FIG. 11 where the valve member closes the inlet port 72 and uncovers the two passages 68 and 70 that constitute the release port of the hydraulic chamber.

When the oil line 60 is initially placed in communication with a high pressure fluid source, the consequent rise of pressure against the pressure-responsive valve member 65 moves the valve member from the first position shown in FIG. 11

'release port of the hydraulic chamber. "At the third position of the valve membershown in FIG. 13, the release port of the hydraulic charnber'56 is still cut off but the inlet port 72 is open to permit the pressurized fluid to flow into the hydraulic chamber through the inlet port 72 and pass the check valve 74 sistance of the previously mentioned coil-spring 45, the force of the coil spring being represented by the arrow 95. Thus in the movement of the valve member 65 from its normal first position to its third position it first closes the release port of the hydraulic chamber and thereafteropens the'inlet port to admit the pressurized fluid. To terminate abraking period the oil line 60 is vented again to alow pressure zone to permit the coil spring 66 to return the pressure responsive valve member 65 to its first position thereby releasing'hydraulic fluid from the chamber 56 to permit the piston to be returned by the spring force 95. i

The, upward movement of the piston54, caused by the ar rival of the pressure responsive valve member 65 at its third position advances the blocking arm 28.until the blocking arm is stopped by the adjustable screw 37. The blocking arm 28 is then hydraulically locked at it advanced position.

I At this advanced position of the blocking arm 28 the rocker arm is free to rock counterclockwise foriperiodi cally fully opening the two'exhaust valves 10, but the 'return clockwise rotation of the rocker arm is blocked by abutment of the rearward face 32 of the rocker arm against the'forward'face 35 of the advanced blocking arm 28. Consequently the two exhaust valves 10 are preventedfrom completely'closing and areeffective for continuous venting of the' engine =cylindcr throughout the duration of the braking eas-a;

It is apparent that the sequence of the interruption of the .closing movements of the various exhaust valves of the engine will be determined by the point in the'normal cycle of the engine at which pressurized hydraulic fluid is supplied to the oil line 60. If the engine is operating at high speed, the time interval of an oscillation of the rocker arm 20 may-be too brief for sufficient pressurized fluid to flow' into'th'e hydraulic chamber to back the blocking arm 28 against the stop screw 37, but in that event, the check valve 74 'traps whatever fluid is in-the hydraulic chamber toprovide a hydraulic lock against retreat lowers the pressurein the chamber with consequent opening of the check valve and automatic replenishment of the leakage through a prolonged braking period. 2

It is apparent that the spring 45 not only biases the piston 54 towards its lower retracted position but also serves to keep the to cause the piston 54 to be advanced in opposition to the reblocking arm 28 in abutment with the outer face of the piston.

l The oil that is released to the outer face of thepiston 54 through the release port of the hydraulic chamber 56 serves to lubricate the outer face of the piston as well as the nose of the blocking arm 28 and other moving parts inside the cylinder head. a The pressurized hydraulic fluidrequired to expand the various hydraulic chambers may be provided by any suitable source of pressurized fluid. The hydraulic fluid may be lubricating oil taken from the crankcase of :the engine and a feature of the'preferr ed practice of the invention is that the required fluid pressure is provided by the oil pump of. the engine. Thus it is not necessary to provide a special reservoir of FIG. 10 shows diagrammatically how a pump may draw oil through a strainer 82; with the outputside of the pump connected to high pressure line 84 and a relief valve 86. The high pressure line 84 communicates with one port of awe position-three connection directional valve 88 that is operable by a solenoid 90 and is normally in the position shown at which it cuts off the high pressure line 84. The previously mentioned oi] line 60 is connected to a second port of the directional valve 88 and has two branches to supply two rows of the hydraulic chambers 56.

It is apparent that at the normal position of the directional valve 88 shown in FIG. 10, the line 60 is vented to a return line 94. When the solenoid 90 is operated, however, the directional valve 88 places the high pressure line 84 in communication with the oil line 60 to cause the engine to function as a brake. Thus the directional valve 88constitutes a control meanshaving one position to initiate, a braking period and a second position to terminate the braking period.

The invention has been described as utilizing the exhaust valves for the braking operation. It is obvious that the same mechanism can be used to hold theintake valves partially open.

My description in specific detail of the presently preferred embodiment of the invention will suggest, various changes, substitutions and other departures of my disclosure within the spirit and scope of the appended claims.

I claim: r

1. In an internal combustion engine whereinan exhaust or intake valve of a cylinder of the engine is-biase'd to closed position and in the normal operation of the engine is periodically forced to open position and wherein means exterior of the cylinder is operatively connected to the valve to advance when the valve opens and to retract when the valve closes, the combination therewith of means to cause the vale to stay at least partially open to retard the engine throughout a braking period, said causing means comprising:

blocking means adjacent said exterior means to advance from a normal retracted position along the path of advance and retraction of the exterior means to a predetermined advanced position to block the retraction of the exterior means thereby to block closing of the valve;

hydraulic means including an expansile hydraulic chamber to advance the blocking means in response to expansion of the chamber and vice versa;

.a fluid passage in communication with the hydraulic chamber;

means to place said fluid passage in communication with a source of pressurized hydraulic fluid continuously throughout the braking period for expansion of the chamber to a predetermined extend to advance the blocking means to the predetermined extent; and

a check valve to prevent reverse flow from the chamber throughout the braking period, whereby when a braking period is initiated and the hydraulic "chamber fails to expand to the predetermined extend on the first advance of thecxterior means, successive increments of hydraulic fluid flow into the hydraulic chamber on successive advances of the exterior means until the accumulated fluid in the hydraulic chamber advances the blocking means to the predetermined extent. a

2. A combination as set forth in claim 1 which includes a stop in the path of said blocking means to limit the advance of the blocking means to said predetermined extent.

3. In an internal combustion engine wherein an exhaust valve or an intake valve of a cylinder of the engine is biased to closed position and means normally opens the valve periodically in timed relation to the rotation of the crank shaft for normal operation'of the engine. the combination therewith of means to cause the valve to stay at least partially open to retard the engine throughout a braking period, said causing means comprising:

blocking means movable from a retracted position to an advanced position to block the closing movement of the valve, said blocking means being retractable to free the hydraulic fluid or to provide a special power actuated pump. 75 valve of the cylinder for closing action;

the inlet port.

a chamber having a movable wall to advance said blocking means in response to rise in fluid pressure in the chamber and vice versa; and

means to supply said chamber with pressurized fluid continuously throughout a braking period to keep said blocking means advanced continuously-throughout the braking period to' block the closing movement of the valve continuously throughout the braking period.

4. A combination as set forth in claim 3 in which said chamber has an inlet port and a release port; and which includes:

A means to place said inlet port in communication with a high pressure fluid source and in communication with a low pressure zone selectively; and

valve means to control said two ports, said valve means being operable to close said release port and to open said inlet port when the inlet port is in communication with the high pressure fluid source thereby to initiate a braking period, said valve means being operable to open said release port to terminate the braking period.

5. A combination as set forth in claim 4 in which said valve means is movable from a normal first position through a second intermediate position'to a third position and vice versa,'the valve means at'its first position closing the inlet port and opening the release port,'the valve member at its second position closing both ports, the valve means at its third position opening the inlet port and closing the release port whereby when the valve, means move from "its first position to its third position it first closes the release port and then opens 6. A combination as set forth in claim Swhich includes a passage leadingto the hydraulic chamber and means toplace said passage in communication with a high pressure fluid sourceand a low pressure zone selectively; and in which said valve means is responsive to pressure in said passage for movement of the valve means from its normal first position to its third position in response to rise in pressure and vice versa.

7. A combination as set forth in claim 3 which includes a check valve to control flow of fluid through said inlet port into the hydraulic chamber.

8. A combination as set forth in claim 4 in which said movable wall is a piston; and in which said inletport, said release port and said valve means are incorporated in the piston.

9. A combination as set forth in claim 3 in which said valve of the cylinder is normally periodically opened by a rocker arm; and in which said blocking means advances to a position to block the return movement of the rocker arm.

10. A combination as set forth in claim 9 in which said blocking means is adjacent the rocker arm and is pivoted to rock on the same axis as the rocker arm, the rocker arm and the blocking means having portions'for mutual abutment at the advanced position of the blocking means.

ll. A combination as set forthin claim 10 in which a portion of said blocking means extends into. the path of movement of said movable wall for actuation of the blocking means by the movable wall; and which includes spring means urging said portion of the blocking means against the movable wall to contract the hydraulic chamber in the absence of fluid pressure therein and to cause the blocking means to follow the movements of the movable wall.

12. A combination as set forth in claim 3 which includes adjustable means to limit the advance of the blocking means to determine the degree to which the valve is held open throughout the braking period.

13. In an internal combustion engine wherein a plurality of cylinders of the engine have a. corresponding plurality of valves either for exhaust or intake .and a corresponding pluralityof operating means to operate the valves in sequence for normal engine operation by first advancing to force the valves open in opposition to the valve springs and then retreating to permit the valve springs to close-the valves, the combination therewith of engine retarder means to keep the valves at least partially open continuously throughout a braking period, said f engine retarder means comprising:

ing a movable wall for expansion and contraction of the effective volume of the hydraulic chamber, each of said movable walls being biased to contract the volume of the hydraulic chamber and-being operatively connected to the corresponding blocking means for advance thereof in response to expansion of the volume of the hydraulic chamber, each of said hydraulic chambers having an inlet port and a release port;

a corresponding plurality of pressure-responsive valve means to control said ports of the respective hydraulic chambers;

passage means to supply hydraulic fluid to said chambers and to provide'fluid pressure for operation of said valve means;

a source of pressurized hydraulic fluid; and

a control valve for said passage means having a normal first position to place the passage means in communication with a low pressure zone to avoid pressure in the passage means, and having a second portion to cut ofi the low pressure zone and to place the passage means in communication with the source of pressurized hydraulic fluid-'for a braking period, each of said pressure-responsive valve means having a normal first position cutting off the corresponding inlet port and opening the corresponding release port, each of said pressure-responsive valve meanshaving a second portion cutting off both of the corresponding ports, each of said pressure-responsive valve means having a third position to cut off the corresponding release portantl to open the corresponding inletport, each of said valve means being movable from its first position through its second position to its third positionin response to rise in pressure in said passage means and vice versa, whereby placing said control valve at its second position operates all of the pressure-responsive valve means to cause expansion of said hydraulic chambers in sequence as permitted by the sequential opening of the valves of the cylinders, thereby causing the blocking meansto advance in sequence to their blocking positions and thereafter to maintain their blocking positions to keep the valves of the cylinders at least partially open continuously throughout a braking period, the braking period being terminated by return of the control valve to its normal setting to drop the pressure in the passage means with consequent operation of said pressure responsive valve means for contraction of the hydraulic chambers and consequent retraction of the blocking means.

14. A combination as set forth in claim 13 in which said plurality of operating means comprises a plurality of cam-actuated rocker arms that rock in one direction to open the valves of the cylinders and rock in thereturn direction to permit the valves to close; and in which said blocking means blocks-the return rocking movements of the rocker'arms. 7

15. A combination as setforth in claim 14 in which each of said blocking means is adjacent the corresponding rocker arm and is pivoted to rock on the same axis as the rocker arm;- and in which the rocker arm and the blocking means have portions for mutual abutment at the advanced position of the blocking means. I

16. A combination as set forth in claim 15 in which a portion of each of said blocking means extends into the path .of

movement of the corresponding movable wall for actuation of the blocking means by the movable wall; and which includes means to follow the movements of the movable wall.

17. A combination as set forth in claim 13 in which each of said movable walls is a piston and in which the corresponding pressure-responsive v valve means together with the corresponding inlet vah/ e-and the corresponding release valve are incorporated in the piston.

18. A combination as set forth in claim 13 which includes a corresponding plurality of check valves to prevent reverse flow from the hydraulic chambers to the passage means.

19. A combination as set forth in claim 13 which includes a corresponding plurality of adjustable stop means to limit the advance of the blocking means respectively'to determine the degree to which the valves of the cylinders are opened during a braking period.

20. In an internal combustion engine wherein a plurality of cylinders of the engine have a corresponding plurality of valves either for exhaust or intake with corresponding valve springs and a corresponding plurality of operating means to operate the valves in sequence for normal engine operation by first advancing to force the valves open in opposition to the valve springs and then retreating to permit the valve springs to close the valves, the combination therewith of means to cause all of said valves to stay at least partially open continuously during a braking period to retard the engine, said means comprising: e

a corresponding plurality of normally retracted blocking means, each adapted to advance from a retracted position to an advanced position to block closing movement of the corresponding valve; means operative to apply force to all of said blocking means continuously throughout a braking period to urge the blocking means to advance, said force being insufficient to overcome the resistance of the valve springs whereby the blocking means advance only when permitted to do so by the movement of the corresponding valves to their open positions; and

means operative to prevent retraction movement of the blocking means by the valve springs throughout a braking period, whereby the blocking means advance in sequence at the beginning of a braking period as permitted by the opening of the valves in sequenceand then remain advanced to keep the valves at least partially open continuously until the end of the braking period.

21. A combination asset forth in claim 20 in which said means to apply force to all of the braking means comprises hydraulic means.

22. A combination as set forth in claim 21 in which said means to prevent retraction movement of the blocking means bythe valve springs throughout a braking period comprises a corresponding plurality of hydraulic locking means.

23. A combination as set forth in claim 20 in which said means to apply force to all of the blocking means comprises:

a corresponding plurality of hydraulic chambers having movable walls to advance the blocking means; and

means to place all of said hydraulic chambers in communication with a source of pressurized fluid.

24. A combination as set forth in claim 23 in which said means to prevent retraction movement of the blocking means by the valve springs comprises a corresponding plurality of check valves to prevent return flow of the hydraulic fluid from the hydraulic chambers. 

